The present invention relates to valve operating apparatus for an internal combustion engine. More particularly, the invention involves valve operating apparatus of the type including a camshaft rotatably driven by the engine, a plurality of cams on the camshaft for operating the intake or exhaust valves according to a selected mode of operation, and pivotably mounted rocker arms for opening and closing the valves in response to rotation of the cams.
In valve operating devices of the type described, driver rocker arms operably connected to the valve or valves to be operated, and free rocker arms, independent of the valves, are disposed adjacent each other and are operable in different modes by rotation of the cams. A selective coupling carried by the rocker arms are hydraulically actuable for selectively interconnecting or disconnecting the respective rocker arms according to the desired mode of valve operation.
In such valve operating device, it is general practice to select relative valve diameters, valve timings, and valve lifts in view of a high-speed range for higher engine output power. However, if an intake valve is actuated at constant valve timing and valve lift throughout a full engine speed range from low to high speeds, then the speed of flow of an air-fuel mixture into the combustion chamber varies from engine speed to engine speed since the amount of air-fuel mixture introduced per unit time varies from engine speed to engine speed. At low engine speeds, the speed of flow of the air-fuel mixture is lowered, the charging efficiency is lowered, and the air-fuel mixture is not mixed and atomized appropriately in the combustion chamber, resulting in slow combustion therein. Therefore, no stable combustion is achieved, and the engine speed becomes irregular during the low-speed operation. Consequently, the combustion efficiency, the fuel economy, and the knocking margin lowered are all reduced.
One solution to the above problem is suggested in Japanese Laid-Open Patent Publication No. 59-226216. According to that solution, some of the intake and/or exhaust valves remain closed when the engine operates at a low speed, whereas all of the intake and exhaust valves are operated, i.e., alternately opened and closed, during high-speed operation of the engine. Controlling the valves differently in low- and high-speed ranges, as suggested by this reference, is not totally dispositive of the problem, however. For example, if the valve control were effected in different modes in more speed ranges, the engine output power would be increased, stable low-speed operation would become possible, and the fuel economy would be improved.
In the above-referred prior art valve operating device, moreover, hydraulic passages for supplying hydraulic pressure respectively to the selective coupling means are defined by dividing the inner space of the rocker shaft by means of a steel ball that is forcibly inserted into and securely positioned in the rocker shaft. With such a structure, the hydraulic pressure must be supplied from the axially opposite sides of the rocker shaft, and hence the hydraulic pressure supply circuit for the valve operating device for use in a multicylinder internal combustion engine becomes complex.
It is to the amelioration of these problems, therefore, that the present invention is directed.